Question: Which of these numbers is prime? ${25,\ 33,\ 63,\ 64,\ 71}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 25 are 1, 5, and 25. The factors of 33 are 1, 3, 11, and 33. The factors of 63 are 1, 3, 7, 9, 21, and 63. The factors of 64 are 1, 2, 4, 8, 16, 32, and 64. The factors of 71 are 1 and 71. Thus, 71 is a prime number.